The so-called quantum computer is of considerable interest for the development of calculating machines. In quantum computers, logic operations are performed on the basis of quantum states and by selectively Influencing such states. Quantum states may be described as resulting from the superposition of a large number of normalized, orthogonal base states, whose weighting is accomplished by probability masses which are definable by measuring processes. In quantum computers, the fundamental unit that corresponds to the binary bit, i.e., the fundamental unit for all arithmetic operations in traditional electronic computers, is the “quantum bit” (qubit). The quantum bit can be represented, in this context, as a linear, weighted, superposed state of two basic functions, namely of the basic functions “0” and “1”. The basic functions correspond to the classic values 0 and/or 1 of the bit. The complex coefficients correspond to the weights with which the basic functions participate in the superposed state. The reference V. Vedral et al., Basics of Quantum Computation, Progr. In Quantum Electronics 22, 1 (1998), purportedly describes the fundamentals of the quantum computer.
An example of such a quantum state, which can be described as a superposition of two orthogonal basic functions, is the polarization state of light. Quantum bits can, therefore, be implemented as the polarization states of individual photons and as logic operations resulting from manipulation of the photons or photon pairs. As mathematical-topological equivalents, quantum bits are represented on the Bloch sphere and polarization states on the Poincare sphere as surface points.
By using light as the basis for a quantum computer, one derives the benefit of being able to selectively influence the polarization state of a single photon, emitted from a photon-pair source.
Another property of light, in this connection, may be that polarization states are correlated within one photon pair stemming from a single decay. The photon pair is quantum mechanically in a so-called “transposed state”, which can be used as the starting state for the quantum computer.
A photon pair source is produced, for example, by an optically non-linear crystal in conjunction with an intensive optical pump light source, usually a laser light source. Under suitable geometric conditions, a single photon of the pump light source decays with a certain probability, while retaining energy and pulse, into two energy quanta or photons. The reference H. Paul, Nonlinear Optics, vol. 2, page 94 ff, Berlin 1973, purportedly describes this phenomenon as parametric fluorescence. The fluorescent light is emitted with two main frequencies or wavelengths, which differ depending on the excitation geometry, in defined spatial directions, relatively to the direction of propagation of the pumping beam. The two fluorescing photons are emitted virtually simultaneously, i.e., within a time period of about 10 femto-seconds, and, depending on the type of nonlinear crystal and the excitation geometry, in the same or in different spatial directions. The polarization of the fluorescing photons is thereby established. The physical properties of the two photons of the parametric fluorescence are linked to one another by a number of secondary conditions. In the case of the transposed photons, it may be a question, quantum mechanically, of a single state in which two photons reside inseparably, measurements at one of the photons allowing precise information to be obtained regarding the physical properties of the corresponding other photon.
The reference German Patent Application No. DE 198 23 849.5 purportedly describes a method and a device for optionally generating individual photons or photon pairs in an optical channel, by way of which a photon pair, in a quantum mechanically transposed state, is able to be selectively spatially separated, or propagated colinearly, as a pair. In purported accordance with the reference German Patent Application No. DE 198 23 849.5, from a photon pair source, photon pairs are generated in a quantum mechanically transposed state, and one photon of the pair is coupled into one partial path of rays of the device, respectively, the two partial paths of rays being reunited at a beam splitter and being directed into two common output channels. Positioned in the first partial path of rays is an interferometer, whose interferometer arms have optical path lengths δlF and δlS. Positioned in the second partial path of rays is an optical delay path of optical length δl. Using means for varying optical path lengths δlF, δlS and/or δl, these path lengths are adjustable such that photon pairs are generated which, as the result of interference, propagate co-linearly or separately in the output channels of the beam splitter. The adjustment is made, for example, via the measurement of the coincidences or spatial coincidences between the output channels.
The separation of the pairs in the context of the system discussed in the reference German Patent Application No. DE 198 23 849.5 is believed to be based on the quantum state of the one photon being split into two preferably separate probability distributions in the state space which have different, namely orthogonal polarization, and are spatially separate from one another. This probability distribution is achieved in that the photon propagates through a polarizing interferometer, which, for example, is a double-refractive crystal or an interferometer, whose design includes polarizing beam splitters aligned at less than 45° to the polarization of the photon. Due to the different optical path lengths in the interferometer arms, a photon is generated. The probability that it resides in the local space has two maxima, which propagate at the speed of light along the same light path and include states having orthogonal polarization. Such a photon, propagating in the z-direction, is shown, for example, schematically in FIG. 1. At Z1, the wave packet is horizontally linearly (y-direction) and, at Z2, vertically linearly polarized.
In this quantum state, the probabilities of finding the photon at Z1 and Z2 are differently polarized may be a drawback. Since it is always only indistinguishable photons which interfere with one another, the two residence probability regions around Z1 and Z2 should have the same polarization, to be optimally further processed and to be brought into interference with the photon in the second partial path of rays. The polarizations of the two regions Z1, Z2 can be compensated or made more alike in that the photon in the first partial path of rays propagates through a linear polarizer, which is situated at less than 45° with respect to the x- and y-axis. Here, half of the correlated photon pairs may be lost in this manner.